1. Field of the Invention
The present invention relates to a nitride semiconductor laser diode using a nitride semiconductor represented by AlxInyGa1-x-yN (0 x 1, 0 y 1, 0 x+y 1).
2. Description of Related Art
A semiconductor laser diode using a compound semiconductor is widely used in applications such as optical disc systems capable of recording and/or reproducing large volume, high-density information. Meanwhile, a new application for semiconductor laser diodes which will provide a full color display with a combination of blue, green, and red semiconductor lasers is expected to be realized.
Among blue, green, and red of the three principal colors of light, blue and red semiconductor laser diodes are already in practical use with employing Group III-V compound semiconductors such as InAlGaN and AlInGaP. In contrast, as for a green laser, a laser device capable of emitting a green beam has been developed by converting wavelength using second harmonic generation (SHG). However, a direct green laser diode capable of directly emitting a green beam has not yet been put into practical use.
As for a semiconductor laser diode capable of directly emitting green light, a laser diode using a Group II-VI compound semiconductor was reported around 1993, but because of its poor reliability at high current condition, its practical use was not realized. For this reason, in recent years realization of a semiconductor laser diode capable of directly emitting green light with a use of a Group III-V nitride semiconductor is expected.
As for a Group III-V nitride semiconductor, a semiconductor laser diode using a light emitting layer made of InxAlyGa1-x-yN (0<x, 0 y<1, 0<x+y<1), particularly made of InGaN, (hereinafter referred simply as “InGaN light emitting layer”) and capable of emitting ultraviolet to blue light is already in practical use (Patent Reference 1 etc.). When the In content in the InGaN light emitting layer is increased, the band gap becomes smaller and the emission wavelength becomes greater and emission of green light becomes possible. However, in the case where the InGaN light emitting layer is grown by vapor phase epitaxy, the lattice mismatch with respect to GaN layer which is an underlayer thereof, becomes greater with increase of In content, and InGaN layer itself becomes chemically unstable and phase separation tends to occur. For this reason, the realization of a semiconductor laser capable of emitting green light, by way of increasing the In content in the InGaN light emitting layer is not easy.
In Patent Reference 2, a semiconductor laser diode capable of emitting light in a wide range of wavelengths including green is proposed, in which InGaNP, obtained by a part of Group V element in InGaN substituted with P is used as the light emitting layer. According to Patent Reference 2, using InGaNP as the light emitting layer enables P, which is a Group V element having less volatile than N, to combine with In, thus segregation of In can be prevented. In addition, bowing effect of band gap due to the change in the content of P is large, so that the content of In necessary to obtain a desired emission wavelength can be reduced compared to that in a conventional InGaN light emitting layer. Also, Patent Reference 3 proposes an addition of an impurity such as Mg, Be, C, or Si to InGaNP light emitting layer to prevent separation of crystalline system in the InGaNP light emitting layer.
In contrast, recently, the inventors of the present invention improve the crystal quality of the InGaN light emitting layer having a high content of In by optimizing its growth condition and succeeded in producing continuous lasing at room temperature up to 515 nm in a semiconductor laser diode using an InGaN light emitting layer, and reported in Non-patent Reference 1. The green semiconductor laser exhibited output power of 5 mW at 25° C. and estimated operating life at room temperature was 5000 hr or greater.    Patent Reference 1: WO 2002-05399    Patent Reference 2: JP 2002-26459A    Patent Reference 3: JP 2002-84040A    Non-patent Document 1: T. Miyoshi et. al., “510-515 nm InGaN-Based Green Laser Diode on c-Plane GaN substrate”, Applied Physics Express 2(2009), 062201.